NGC 6397

NGC 6397
A Hubble Space Telescope (HST) image of NGC 6397. Credit: HST/NASA/ESA.
Observation data (J2000 epoch)
Class	IX
Constellation	Ara
Right ascension	17h 40m 41.36s[1]
Declination	−53° 40′ 25.3″[1]
Distance	7.2 kly[2] (2.2 kpc)
Apparent magnitude (V)	+6.68[1]
Apparent dimensions (V)	32′.0
Physical characteristics
Mass	(presently unknown kg)
Radius	34 ly[3]
VHB	14.2
Estimated age	13.4 ± 0.8 Gyr[2]
Notable features	Second closest globular to Earth
Other designations	GCl 74,[1] Lacaille III.11, Dunlop 366, Bennett 98
See also: Globular cluster, List of globular clusters

NGC 6397 is a globular cluster in the constellation Ara. It is located about 7,200 light-years from Earth, making it one of the two nearest globular clusters to Earth (the other one being Messier Object 4). The cluster contains around 400,000 stars,^[2] and can be seen with the naked eye under good observing conditions.^[4]

NGC 6397 is one of the at least 20 globulars of our Milky Way Galaxy which have undergone a core collapse,^[2] meaning that its core has contracted to a very dense stellar agglomeration.

Astronomical research

Estimating the age of the Milky Way

In 2004, a team of astronomers^[2] focused on the cluster to estimate the age of the Milky Way Galaxy. The team consisted of Luca Pasquini, Piercarlo Bonifacio, Sofia Randich, Daniele Galli, and Raffaele G. Gratton. They used the UV-Visual Echelle Spectrograph of the Very Large Telescope to measure, for the first time, the beryllium content of two stars in the cluster. This allowed them to deduce the time elapsed between the rise of the first generation of stars in the entire Galaxy and the first generation of stars in the cluster. They added in the estimated age of the stars in the cluster to arrive at an age for the Galaxy (about 13.6 billion years, which is nearly as old as the universe itself).

Lower mass limit for stars

In 2006, a study of NGC6397 using the Hubble Space Telescope was published that showed a clear lower limit in the brightness of the cluster's population of faint stars. The authors deduce that this indicates a lower limit in mass for stars to develop a core that is capable of fusion, and obtain a value of approximately 0.083 times the mass of the Sun.^[5]

References

1. ^ ^{a b c d} "SIMBAD Astronomical Database". Results for NGC 6397. http://simbad.u-strasbg.fr/Simbad. Retrieved 2006-11-16. 
2. ^ ^{a b c d e} "How Old is the Milky Way ?". Results for NGC 6397. http://www.eso.org/outreach/press-rel/pr-2004/pr-20-04.html. Retrieved 2006-09-26. 
3. ^ distance × sin( diameter_angle / 2 ) = ~34 ly. radius
4. ^ Dunlop, Storm (2005). Atlas of the Night Sky. Collins. ISBN 0-00-717223-0. 
5. ^ "HST analysis of faint stars in NGC6397". Results for NGC 6397. http://hubblesite.org/newscenter/newsdesk/archive/releases/2006/37. Retrieved 2006-09-26.